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[2014 Spring Life Sciences & IBB Regular Seminar]
▶Subject: Mechanisms of the granum thylakoid assembly in germinating Arabidopsis seeds revealed by electron tomography
▶Speaker: Prof. Byung-Ho Kang (University of Florida)
▶Place: Auditorium(1F), Postech Biotech Center
The chloroplast is a type of plastids that is responsible for photosynthesis, producing molecular oxygen and reduced carbon molecules using light energy from the sun. Chloroplasts have thylakoids where the proteins and pigments that capture photons and convert them into chemical energy are embedded. During germination, proplastids in the seed develop into chloroplasts quickly so that new seedlings become autotrophs. There have been electron tomography (ET) studies of mature chloroplasts and young chloroplasts in the shoot apical meristem. In this study, we have examined the rapid thylakoid biogenesis in germinating Arabidopsis cotyledons using ET. After 24 hours after imbibition (HAI), proplastids containing membrane tubules are observed in the perinuclear region. In 36 HAI cotyledon cells, plastids expand and their stromal ribosome density increases. The plastid inner membrane grows into the stroma and the membrane ingrowths fuse to form vesicular-tubular clusters. These clusters flatten out and extend into the stroma where ribosomes are attached on these lamellae to form “rough thylakoids.” Stacking of thylakoids is first detected in 36 HAI chloroplasts but grana stacking and segregation of grana from stromal thylakoid occurs in 60 HAI chloroplasts. By this time point, thylakoids grown out from different sites in the inner membrane have fused to permeate through the stroma. Grana stacks emerge from thylakoid branches that are flattened and overlaid on top of neighboring stacks. Ribosomes are attached on the top and bottom surfaces of grana stacks, probably inserting photosynthetic membrane proteins. To investigate molecular players of the thylakoid assembly, we examined developing chloroplasts in the cotyledons of fzl, and rh3-4 mutants that exhibit aberrant thylakoid structure and delayed stromal ribosome biogenesis. Individual thylakoids failed to interconnect and numerous vesicles are observed in the fzl mutant chloroplasts. In rh3-4 mutant at 36 HAI, multi-lamellae accumulate in the stroma but they rapidly transform into grana by producing “flaps” that intercalate into stacks once stromal ribosomes are synthesized. These results indicate that FZL is involved in thylakoid membrane fusion and that stroma ribosomes directly insert photosynthetic proteins into thylakoid membrane that are required for the assembly of grana stacks.
▶Inquiry: Prof.Youngsook Lee(279-2296)
* This seminar will be given in English